Sintered ceramic articles which are made of mixtures of borides and nitrides and other refractory components have been known. Such ceramic articles include ceramic cutting tools, nozzles, resistive refractory boats and similar articles, use of which require hardness, toughness, resistance to wear and to corrosion by molten metals at high temperatures. In the case of refractory boats, electrical conductivity of the sintered ceramic product is an additional requirement.
Conventional boride based ceramic compositions are known to have high hardness, toughness, corrosion resistance, but often have densities which are below the most advantageous density value. An example of compositions for hot pressed and sintered, hard refractory articles is U.S. Pat. No. 3,256,103 issued to Martin A. Roche et al. on Jun. 14, 1966. The mixture for the refractory articles contains titanium boride, boron nitride and titanium nitride. The density of these articles however, have been found to be insufficiently high to withstand prolonged wear and corrosion by molten metals.
Sintered hard ceramic articles, and a method and apparatus for compacting such articles are described in U.S. Pat. No. 4,394,170 issued to Akira Sawaoka et al. on Jul. 19, 1983. The compositions of Sawaoka et al. include two types of boron nitrides, several other nitrides, borides, oxides and a metal. Sawaoka et al. utilizes a press described therein to compact and hot-press the articles made of compositions taught in U.S. Pat. No. 4,394,170. Hiroaki Nishio et al. in U.S. Pat. No. 4,933,308 issued Jan. 12, 1990 describe compositions to obtain sintered ceramic articles having high strength and high fracture toughness. The notable components of the compositions of Nishio et al. are titanium diboride, up to 30 wt. % silicon carbide and 2-20 wt. % zirconium dioxide. Yttria, magnesia, ceria are also added in amounts related to the amount of zirconia present together with small amounts of other borides and carbides, which are added to control grain growth. The articles prepared and cast by Nishio et al. are sintered in two stages; first at 1600.degree.-2000.degree. C. in a non-oxidizing atmosphere, then the articles are further densified by hot isostatic pressing (HIP). Densities of sintered articles range between 95-99% of theoretical densities.
Hajime Saito et al. in U.S. Pat. No. 5,185,112 issued on Feb. 9, 1993 describe sintered ceramic articles obtained by mixing titanium boride with chromium and graphite powder and a binder, and compacting the mixture obtained at high pressure at ambient temperature. The articles so obtained are subsequently sintered in a non-oxidizing atmosphere at temperatures up to 2000.degree. C. Chromium carbide and chromium boride embedded in the titanium diboride matrix are other notable components of the sintered articles made of compositions of Saito et al.
Ceramic boats self-heated by resistance heating are often used for vaporizing metals, more particularly aluminum and its alloy. It is usual that such boats have an electrically conductive ceramic component to provide the required resistivity. The boat however, also has to be made of a hard wearing, refractory composition which is also resistant to corrosion by molten metal. Most of the conventional compositions referred to hereinabove, are capable of conducting electricity and hence, apart from being utilized in the manufacture of wear-resistant articles, may also be used in the production of self-heated ceramic boats, also known as evaporating boats.
The electrical, chemical and mechanical demands on such boats are very high, resulting in relatively short life-span of the boats. There are conventional methods for recycling spent evaporating boats to production by grinding the boats, and mixing the ground particles with titanium boride, boron nitride, aluminum nitride to obtain the required composition, casting the mixture into billets or ingots and machining the ingots sintered at high temperature to obtain ceramic boats.
In some instances it was found that conventionally manufactured ceramic boats exhibited directional properties, which resulted in reduced lifespan of such boats. Passmore et al. in U.S. Pat. No. 3,803,707 issued on Apr. 16, 1974, teach the casting and pressing of ceramic ingots composed of titanium diboride, boron nitride, and optionally aluminum nitride into cylindrical ingots, and subsequently machining boats from the ingots, such that the longitudinal axis of the boats are at right angles to the longitudinal axis of the cylindrical ingot. It may be seen that the method taught by Passmore et al. may lead to some wastage of relatively costly material. Thus it may be concluded that a ceramic composition that would allow the manufacture of ceramic evaporating boats from cast ceramic ingots which have substantially non-directional properties, could reduce the cost of production and utilization of the ceramic boats.
Klaus Hunold et al. in U.S. Pat. No. 4,528,120 issued on Jul. 9, 1985 describe a ceramic composition for sintered electrically conductive articles having non-directional properties. The components of the composition of Hunold et al. include hexagonal boron nitride, aluminum nitride or silicon nitride and an electrically conductive boride or carbide. The cast article made of the composition of Hunold et al. is subsequently sintered in two stages: first sintering in an inert gas at about 800.degree. C., then encasing the cast articles in a refractory metal and subjecting the encased ceramic article to hot iso-static pressing (HIP) in an inert atmosphere.
It is to be noted that most of the ceramic articles made of the above described conventional compositions have densities which deviate from the theoretical density in varying degrees. Densities which are close to the theoretical density are usually attained only by means of costly high pressure process steps applied at high temperatures.
It may be seen from the foregoing that there is a need for compositions that provide dense sintered ceramic articles by relatively inexpensive production process steps.